PSI - Issue 33

M.P. Tretyakov et al. / Procedia Structural Integrity 33 (2021) 1089–1094 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Keywords: Portevin-Le Chatelier effect; jerky flow; stiffness of the loading system; postcritical deformation; tension.

1. Introduction Ensuring high reliability of the created structures and preventing industrial accidents is largely determined by the availability of the necessary information about the physical, mechanical and strength properties of the materials used, taking into account the influence of real mechanical and thermal external influences that cause the course of multilevel processes of deformation and destruction. Failure of structural elements is often caused by elastoplastic deformation of the material, accompanied by a change in its structure and physical and mechanical properties during loading. The effects of intermittent flow lead to a significant decrease in strength and ductility, and reduce the surface quality of the material. In this case, the processes of strip formation and spontaneous macroscopic localization of plastic flow lead to the appearance of a difference in thickness of parts, the appearance of concentration and defects, and, as a consequence, to the subsequent macro fracture. It seems promising to obtain and systematize experimental data on the regularities and features of discontinuous fluidity under various physical and mechanical effects based on the analysis of the kinetics of deformation fields in the process of initiation and development of deformation macro localization and instability of plastic flow using the example of Al-Mg alloys. Yilmaz (2011), Aguirre et al. (2004), Tretyakova and Wildemann (2016, 2019) consider the study of these issues of inelastic behavior of materials. Analysis of structures and structures within the framework of the concept of fracture as a result of loss of stability of inelastic deformation processes involves the study of the basic laws of the mechanical behavior of materials at the final stage of deformation - the stage of deformation softening, or post-critical deformation. A manifestation of this stage is a falling section in the deformation diagram. The theoretical and experimental study of the basic laws of this phenomenon creates conditions for a more adequate prediction of the conditions of destruction of deformable bodies and analysis of the possibilities of controlling the processes of destruction and are considered by Vildeman et al. (1997) Tretyakov et al. (2016, 2018). It is at the supercritical stage of deformation that the formation of macro fracture conditions occurs, which, in contrast to the traditional concepts that determine the use of force or deformation strength criteria, are not uniquely associated with the stress-strain state at the point of the deformed body. As shown by Videlman and Tretyakov (2013), the loading system plays a key role in the transition from the stage of equilibrium damage accumulation to the unstable fracture stage. The mechanical behavior of materials at the supercritical stage of deformation has the following regularities: the processes of structural destruction and cracking are reflected in the deformation diagram, leading to its nonlinearity, and at the final stage are the cause of softening; fracture resistance at the supercritical stage of deformation corresponding to the descending branch of the deformation diagram depends on the rigidity of the loading system; the diagram breaks off at the highest point only at zero rigidity of the loading system, i.e. under “soft” (force) loading, under “rigid” (kinematic) loading, the relationship between load and displacement is represented by complete diagrams when the tensile force is reduced to zero load value. In the general case, each point on the falling branch can correspond to the moment of loss of bearing capacity, depending on the loading conditions. Loss of bearing capacity can be considered as a transition from a stable to a unstable stage of the structural destruction process at the supercritical stage. Experimental data in some cases show the presence of much extended falling sections in the deformation diagrams of the material, corresponding to the softening stage at various types of stress-strain state. It is obvious that ignoring this stage in traditional strength calculations leads to the loss of important information about deformation reserves and survivability of structural elements. Because intermittent flow processes are characterized by the instability of the ongoing deformation processes, a significant dependence of this phenomenon on the rigidity parameters of the loading system is expected. The aim of this work is to study the influence of the rigidity of the loading system with respect to the working part of the specimen on the processes of inhomogeneous intermittent deformation of the Al-Mg alloy at the stages of elastoplastic and supercritical deformation.

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